1. Field of The Invention
This invention relates to wireless communications and, more particularly, to a multiple mode data communication system and method and a forward and/or reverse link control channel structure to support various features.
2. Description of Related Art
Wireless communications systems include conventional cellular communication systems which comprise a number of cell sites or base stations, geographically distributed to support transmission and receipt of communication signals to and from wireless units which may actually be stationary or fixed. Each cell site handles voice communications over a particular region called a cell, and the overall coverage area for the cellular communication system is defined by the union of cells for all of the cell sites, where the coverage areas for nearby cell sites overlap to some degree to ensure (if possible) contiguous communications coverage within the outer boundaries of the system's coverage area.
When active, a wireless unit receives signals from at least one base station or cell site over a forward link or downlink and transmits signals to (at least) one cell site or base station over a reverse link or uplink. There are many different schemes for defining wireless links or channels for a cellular communication system, including TDMA (time-division multiple access), FDMA (frequency-division multiple access), and CDMA (code-division multiple access) schemes. In CDMA communications, different wireless channels are distinguished by different codes or sequences that are used to encode different information streams, which may then be modulated at one or more different carrier frequencies for simultaneous transmission. A receiver can recover a particular information stream from a received signal using the appropriate code or sequence to decode the received signal.
Due to the delay-intolerant nature of voice communication, wireless units in conventional cellular systems transmit and receive over dedicated links between a wireless unit and a base station. Generally, each active wireless unit requires the assignment of a dedicated link on the forward link and a dedicated link on the reverse link. Traditional data applications are typically bursty and, unlike voice communications, relatively delay tolerant. As such, using dedicated links to transmit data is an inefficient use of network resources. Wireless communications systems are evolving that will support a variety of data services, such as wireless web browsing.
In a well known data only evolution of the third generation CDMA standard (hereinafter referred to as 3G-1x EVDO), data is transmitted on the forward link over a time division multiplexed carrier at fixed data transmit powers but at variable data rates. Measured signal to interference ratio (SIR) at the receiver is used to determine a data rate which can be supported by the receiver. Typically, the determined data rate corresponds to a maximum data rate at which a minimum level of quality of service can be achieved at the wireless unit. Higher measured SIR translates into higher data rates, wherein higher data rates involve higher order modulation and weaker coding than lower data rates. To improve system throughput, the system allows the wireless unit with the best channel, and thereby the highest rate, to transmit ahead of wireless units with comparatively low channel quality. On the reverse link, each user transmits data using a code channel, and users transmit autonomously with little or no synchrony with other users. The base station can signal to the users on a forward link common control channel that a received power threshold has been exceeded. In response, the wireless units perform a persistence test to determine whether to increase or decrease the data rate.
In the Universal Mobile Telecommunications System (UMTS), wireless units communicate with a base station over dedicated channels. To provide efficient wireless data communications on the forward link, UMTS uses a shared channel which can be shared by a plurality of wireless units to receive data. To improve system throughput, the system provides the wireless unit with the best reported rate access to the shared channel. On the reverse link, UMTS uses a time-multiplexed CPCH—(common packet channel) which is not completely defined, but proposals have users transmitting data autonomously using a slotted ALOHA technique where users transmit at any time. If the wireless unit does not receive an acknowledgment, then the wireless unit re-transmits after a random integer number of time slots has passed.
A well known evolution of the third generation CDMA standard (hereinafter referred to as 3G-1x EV-DV), is being developed. On the forward link, voice, data and control information (including signaling and protocol information) are transmitted on the same RF carriers using different Walsh codes. On the reverse link, multiple users transmit over the same RF carrier using the designated Walsh code(s) for the supplemental channel (R-SCH). Each user transmits over the R-SCH using the user's unique long code to distinguish the user from other users.
Two fundamental approaches to improve the throughput of the reverse link supplemental channel have been proposed. One approach which evolved from the 3G CDMA standard is based on scheduling, where the user requests access to the supplemental channel, and the base station allocates resources to the user for the transmission of data over the supplemental channel. It appears that fast scheduling can deliver significant gains via higher data rates/shorter frames and hence better aggregate throughput even after considering the higher overheads. Another approach which evolved from 1xEV-DO is based on autonomous wireless unit transmission. The alternative autonomous approach to wireless unit data transmission may be considered with some kind of wireless unit-specific rate supervision by the base station. Harmonization of these two approaches has been suggested to expedite the reverse link development.